LG 42LG50 no backlight or display

[Daveleone]

Hi all! I have an LG 42LG50 that, when plugged in, shows a flashing red LED on the front, then the backlight flashes for a brief moment, then the front LED turns a very dim blue. (similar to this thread: https://www.badcaps.net/forum/showthread.php?t=52947)

Here are the connector pinouts with [expected voltage when on]:

P201
(left pins)
23 N/C
21 BR1 [1.7v]
19 PWR_ON [2.8v]
17 ERROR [0v]
15
13
11 [5.1v]
09 [5.1v]
07
05 [12.3v]
03
01 [16.5v]

(right pins)
24 N/C
22 PWM_DIM [1.4v]
20 ON/OFF [3.2v]
18 ACD [5v]
16 GND
14 GND
12 5V [5.1v]
10 5V [5.1v]
08 GND
06 12V [12.3v]
04 GND
02 16.5V [16.5v]

P204
01 24v
02 24v
03 24v
04 24v
05 24v
06 0v
07 0v
08 0v
09 0v
10 0v
11 BR1 [1.7v]
12 ON/OFF [3.2v]
13 PWM_DIM [1.4v]
14 ERROR [0v]

P205
01 24v
02 24v
03 24v
04 24v
05 24v
06 0v
07 0v
08 0v
09 0v
10 0v
11 BR1 [1.7v]
12 ON/OFF [3.2v]

I've checked the voltages at the power supply during power on, during the time when the backlight flashes, and after the backlight flash. As a convention, if I've listed a single voltage, that means that voltage is stable throughout the startup process. If I list multiple voltages it means the voltage changes during the cycle, with the format Vpower on->Vbacklight flash->Vafter backlight flash. I've underlined the values I think are suspect.

P201 (top to bottom)
(left pins)
23 0v
21 0v->1.87v (but quickly drops to 0.66v)->3.3v
19 2.8v
17 0v->0v->3.3v
15 0v
13 0v
11 5.2v
09 5.2v
07 0v
05 12.5v
03 0v
01 16.1v

(right pins)
24 0v
22 0v
20 3.4v->1.7v->3.4v
18 5.0v
16 0v
12 5.2v
10 5.2v
08 0v
06 12.5v
04 0v
02 16.1v

P204 (left to right)
01 24.8v
02 24.8v
03 24.8v
04 24.8v
05 24.8v
06 0v
07 0v
08 0v
09 0v
10 0v
11 0v->1.6v (but quickly drops to 0.66v)->3.3v
12 3.3v->1.7v->3.3v
13 0v
14 0v->0v->3.3v

P205 (top to bottom)
01 24.8v
02 24.8v
03 24.8v
04 24.8v
05 24.8v
06 0v
07 0v
08 0v
09 0v
10 0v
11 0v->1.8v (but quickly drops to 0.66v)->3.3v
12 3.3v->1.7v->3.4v


My thoughts and questions...

• Based on the stable voltages at P201 pins 2,4,10,12, and 18, and at P204/205 pins 1-5, I think the power supply is ok.
• I took apart the panel and was able to see all of the CFL tubes light up momentarily, so the CFL tubes themselves seem ok. (I've attached a 2 jpg sequence of the bulbs powering up)
• Obviously, the 3.3v on ERROR (P201 pin 17, P204 pin 14) means something went wrong.
• Why does BR1 (P201 pin 21, P204 pin 11, P205 pin 11) go to 1.7v, then drop to 0.66v, and finally become stable at 3.3v (expected stable value is 1.7v)?
• Why does PWM_DIM (P201 pin 22, P204 pin 13) remain 0v? Why does it never rise to the expected 1.4v?
• Why does ON/OFF (P201 pin 20, P204 pin 12, P205 pin 12) go to 3.4v, then drop to 1.7v, then go back to 3.4v (expected stable value is 3.4v, so maybe that's ok since it is eventually stable at 3.4v)?
• How to figure out if the problem is the main board, the t-con, the CFL driver/ballast boards, or the LCD panel itself?

What would you recommend as next steps in hunting down the source of the ERROR? As always, your help is greatly appreciated!

Thanks,
--Dave

[Andrew F. Ali]

Re: LG 42LG50 no backlight or display

Check the T-CON Bd. The little white fuse might be open due to a shorted component on the T-CON Bd. Are there a blue LED on the Inverter section that comes on and then go off? Also are there any blue LEDs coming on, on the T-CON Bd.?

[budm]

Re: LG 42LG50 no backlight or display

You need to remove those two black shield from the inverter boards so we can see what they look like and how they are setup.

[vinceroger69]

Re: LG 42LG50 no backlight or display

faulty inverter transformer causing two second to black fault?

[neilc6]

Re: LG 42LG50 no backlight or display

Though not the same model, check this thread https://www.badcaps.net/forum/showthread.php?t=62591

[Daveleone]

Re: LG 42LG50 no backlight or display

Thanks for the replies! Somehow even though I subscribed to the thread I didn't get notified. Anyways, I'll take some pics of the inverter boards sans shields tonight and post them, and I will look for/check the fuse on the t-con. Thanks!

[CapLeaker]

Re: LG 42LG50 no backlight or display

Check and compare the output winding of the ccfl transformers on the inverter board with your DMM in Ohm mode. They have to be within 3% of each other.

[Daveleone]

Re: LG 42LG50 no backlight or display

You need to remove those two black shield from the inverter boards so we can see what they look like and how they are setup.

Pics attached.

Check and compare the output winding of the ccfl transformers on the inverter board with your DMM in Ohm mode. They have to be within 3% of each other.

Could you please point me to the points I should test in the attached photo?

Check the T-CON Bd. The little white fuse might be open due to a shorted component on the T-CON Bd. Are there a blue LED on the Inverter section that comes on and then go off? Also are there any blue LEDs coming on, on the T-CON Bd.?

Andrew - In the attached picture is the correct fuse the one marked FP1 on the attached image? That fuse tests GOOD. I didn't see any LEDs turn on on either the T-Con or the Inverter boards. The only LED I see is a green LED on the main board. It stays lit as long as power is applied to the TV.

Thank you all for all the help!
--Dave

[Daveleone]

Re: LG 42LG50 no backlight or display

Check and compare the output winding of the ccfl transformers on the inverter board with your DMM in Ohm mode. They have to be within 3% of each other.

Are the desired test points the copper pads next to each CFL connector? If so, the resistance between those are all in the range of 3.28-3.31kohms.

[budm]

Re: LG 42LG50 no backlight or display

1) All the fuses on both boards are good?
2) http://www.shopjimmy.com/cmo-27-d020...verter-kit.htm what Voltages do you have on all the pins of the connector CN1 when the backlights go off? You can see the pin label for each pin so you must report the pin name and follow by the Voltage reading for each pin.

[Daveleone]

Re: LG 42LG50 no backlight or display

1) All the fuses on both boards are good?
2) http://www.shopjimmy.com/cmo-27-d020...verter-kit.htm what Voltages do you have on all the pins of the connector CN1 when the backlights go off? You can see the pin label for each pin so you must report the pin name and follow by the Voltage reading for each pin.

Just to confirm that I'm not missing any - there are 4 fuses on each board, right? They all test GOOD.

After BL shutoff...
CN1 (Master)
VBL 24.8
VBL 24.8
VBL 24.8
VBL 24.8
VBL 24.8
GND 0
GND 0
GND 0
GND 0
GND 0
Status 3.27
E-PWM 3.39
I-PWM 16.0
BLON 3.3

CN2 (Slave)
VBL 24.8
VBL 24.8
VBL 24.8
VBL 24.8
VBL 24.8
GND 0
GND 0
GND 0
GND 0
GND 0
NC 3.27 (not sure if this is ok?)
NC 3.39 (not sure if this is ok?)

[lookimback]

Re: LG 42LG50 no backlight or display

Disconnect the ribbon cable which connects the T-con to the inverter and see if the backlights will work.

[Daveleone]

Re: LG 42LG50 no backlight or display

Disconnect the ribbon cable which connects the T-con to the inverter and see if the backlights will work.

They do!

With the ribbon cable from the master inverter to the T-Con disconnected, the backlight stays lit (The cable from the main board to the T-Con is still connected). Of course, there is no picture on the screen.
What does that mean? Does it mean the the Master & Slave Inverters are ok and we can narrow the root cause down to T-Con, Main Board, or LCD Panel?

[budm]

Re: LG 42LG50 no backlight or display

Disconnect the ribbon cable which connects the T-con to the inverter and see if the backlights will work.

+1
You need to also provide the pin name of that connector that goes to the T-CON board.

[lookimback]

Re: LG 42LG50 no backlight or display

They do!

With the ribbon cable from the master inverter to the T-Con disconnected, the backlight stays lit (The cable from the main board to the T-Con is still connected). Of course, there is no picture on the screen.
What does that mean? Does it mean the the Master & Slave Inverters are ok and we can narrow the root cause down to T-Con, Main Board, or LCD Panel?

I had an LG 42LB5D, which is very similar, and it worked the same. I think I replaced the T-con on mine, but it was so long ago that I can't remember. Someone with a bit more knowledge than me will probably be able to tell you how to determine if it's the T-con or the master inverter.

Main board is probably OK. Power supply is OK. Slave inverter should be OK. This narrows it down to T-con and Master Inverter.

[budm]

Re: LG 42LG50 no backlight or display

That T-CON controls the dimming function.
Since you can remove that cable and able to get the light source to come on, so if the main board, t-con, and panel are OK then you should see pictures on the screen.

[Daveleone]

Re: LG 42LG50 no backlight or display

+1
You need to also provide the pin name of that connector that goes to the T-CON board.

On the T-Con the connector is named CON1
On the Master Inverter, I don't see a name, but the connector pinout:
DIM_OUT
GND
GND
GND
GND
GND
DIM_IN
E1

budm, do you think the Master/Slave inverters are ok? Or could the problem still lie in the Master inverter? If it's for sure the T-Con, main board, or panel, how to determine which?

Thanks guys! Making progress!

[budm]

Re: LG 42LG50 no backlight or display

If you can disconnect that cable to the T-CON board then the backlights come on and stay on then your inverters/lamps are OK. As I indicated, you should now see the pictures on the screen. You TV has what is called dynamic contrast, the T-CON controls the backlight level based on video scenes.

[lookimback]

Re: LG 42LG50 no backlight or display

I was able to find my repair notes and the TV I had ended up having dry solder joints on one of the main board wire connectors.

[Andrew F. Ali]

Re: LG 42LG50 no backlight or display

I suggested looking at the T-CON Bd. in post 32 of this thread. I am still thinking the T-CON Bd has a problem. Yes, probably dry solder joints on the T-CON and Main Bds. as lookimback suggests.